[SCHEDULE 13D/A] Dianthus Therapeutics, Inc. /DE/ SEC Filing

Rhea-AI Filing Summary

Fairmount Funds Management and affiliated entities reported beneficial voting and dispositive power over 4,052,052 shares of Dianthus Therapeutics, Inc. (DNTH), representing 9.9% of the outstanding common stock based on 40,561,085 shares as of September 11, 2025. The position includes 3,307,191 shares of common stock and pre-funded warrants exercisable for up to 744,861 shares subject to a 9.99% beneficial ownership limitation. On September 11, 2025, Fairmount Healthcare Fund II L.P. purchased 600,000 shares in the company’s underwritten public offering at $33.00 per share for aggregate consideration of $19,800,000. Tomas Kiselak resigned from the Board on March 4, 2025, and the Reporting Persons state they currently have no present plans to effect corporate actions, while reserving the right to review or change that position in the future.

Positive

• Material disclosed investment: Fund II purchased 600,000 shares for $19,800,000 at $33.00 per share in the underwritten offering.
• Significant stake: Reporting Persons have voting and dispositive power over 4,052,052 shares, representing 9.9% of outstanding common stock.
• Transparency: Filing updates prior Schedule 13D amendments and details the composition of shares and pre-funded warrants.

Negative

• Ownership capped by warrants: 744,861 pre-funded warrants are exercisable only subject to a 9.99% beneficial ownership limitation.
• Reduced board influence: Tomas Kiselak resigned from the Board on March 4, 2025, and no longer may be deemed to influence company corporate activities.
• Potential limit to increase stake immediately: Beneficial ownership limitation may prevent exercise of warrants into shares beyond the 9.99% threshold.

Insights

Financial Analyst

TL;DR: Fairmount built a near-10% stake via a $19.8M purchase in the September offering, marking a material, disclosed strategic investment.

Fairmount and its related vehicles now have voting and dispositive power over 4,052,052 shares (9.9%). The disclosed $33.00 per-share purchase of 600,000 shares for $19.8 million in the underwritten offering is a concrete capital commitment and increases public visibility into ownership concentration. The position combines outright shares and pre-funded warrants subject to a 9.99% ownership cap, which limits potential further immediate economic/strategic exposure from warrant exercises.

Corporate Governance Expert

TL;DR: The filing signals a significant investor presence but reports no current plans to seek control; a director resignation reduced direct board influence.

The Schedule 13D/A discloses that Mr. Kiselak resigned from the Board on March 4, 2025, expressly noting no disagreement with management. While the Reporting Persons state they have no present plans to pursue actions enumerated under Item 4, they reserve the right to reassess. The combination of disclosed voting/dispositive power and the ability to reconsider strategy is material to governance monitoring for shareholders and the board.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 3)

Dianthus Therapeutics, Inc. /DE/

(Name of Issuer)

Common Stock, $0.001 par value

(Title of Class of Securities)

252828108

(CUSIP Number)

Ms. Erin O'Connor
Fairmount Funds Management LLC, 200 Barr Harbor Drive, Suite 400
West Conshohocken, PA, 19428
(267) 262-5300

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

09/11/2025

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SCHEDULE 13D

CUSIP No. 252828108

1

Name of reporting person

Fairmount Funds Management LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 4,052,052.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,052,052.00

11

Aggregate amount beneficially owned by each reporting person

4,052,052.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

9.9 %

14

Type of Reporting Person (See Instructions)

IA

Comment for Type of Reporting Person:

The securities include (i) 3,307,191 shares of common stock, $0.001 par value ("Common Stock"), and (ii) pre-funded warrants to purchase up to 744,861 shares of Common Stock (the "Pre-Funded Warrants"), the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 40,561,085 shares of Common Stock outstanding as of September 11, 2025, consisting of (i) 32,188,345 shares of Common Stock outstanding as of August 6, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 7,627,879 shares of Common Stock issued in the Company's underwritten public offering on September 11, 2025, as reported in the Company's Current Report on Form 8-K filed on September 11, 2025, and (iii) the 744,861 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation.

SCHEDULE 13D

CUSIP No. 252828108

1

Name of reporting person

Fairmount Healthcare Fund II L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 4,052,052.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,052,052.00

11

Aggregate amount beneficially owned by each reporting person

4,052,052.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

9.9 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The securities include (i) 3,307,191 shares of Common Stock, and (ii) 744,861 shares of Pre-Funded Warrants, the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 40,561,085 shares of Common Stock outstanding as of September 11, 2025, consisting of (i) 32,188,345 shares of Common Stock outstanding as of August 6, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 7,627,879 shares of Common Stock issued in the Company's underwritten public offering on September 11, 2025, as reported in the Company's Current Report on Form 8-K filed on September 11, 2025, and (iii) the 744,861 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation.

SCHEDULE 13D

CUSIP No. 252828108

1

Name of reporting person

Fairmount SPV III, LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

CUSIP No. 252828108

1

Name of reporting person

Peter Evan Harwin

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 4,052,052.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,052,052.00

11

Aggregate amount beneficially owned by each reporting person

4,052,052.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

9.9 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The securities include (i) 3,307,191 shares of Common Stock, and (ii) 744,861 shares of Pre-Funded Warrants, the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 40,561,085 shares of Common Stock outstanding as of September 11, 2025, consisting of (i) 32,188,345 shares of Common Stock outstanding as of August 6, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 7,627,879 shares of Common Stock issued in the Company's underwritten public offering on September 11, 2025, as reported in the Company's Current Report on Form 8-K filed on September 11, 2025, and (iii) the 744,861 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation.

SCHEDULE 13D

CUSIP No. 252828108

1

Name of reporting person

Tomas Kiselak

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

SLOVAKIA

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 4,052,052.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,052,052.00

11

Aggregate amount beneficially owned by each reporting person

4,052,052.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

9.9 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The securities include (i) 3,307,191 shares of Common Stock and (ii) 744,861 shares of Pre-Funded Warrants, the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 40,561,085 shares of Common Stock outstanding as of September 11, 2025, consisting of (i) 32,188,345 shares of Common Stock outstanding as of August 6, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 7,627,879 shares of Common Stock issued in the Company's underwritten public offering on September 11, 2025, as reported in the Company's Current Report on Form 8-K filed on September 11, 2025, and (iii) the 744,861 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation.

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, $0.001 par value
(b)	Name of Issuer: Dianthus Therapeutics, Inc. /DE/
(c)	Address of Issuer's Principal Executive Offices: 7 Times Square, 43rd Floor, New York, NEW YORK , 10036.
Item 1 Comment: This Amendment No. 3 amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission on September 21, 2023, Amendment No. 1 filed on January 24, 2024, and Amendment No. 2 filed on April 1, 2024 (as amended, the "Statement" or the "Schedule 13D") by the Reporting Persons with respect to the common stock, $0.001 par value ("Common Stock"), of Dianthus Therapeutics, Inc. (the "Company"). Unless otherwise defined herein, capitalized terms used in this Amendment No. 3 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged. The address of the principal executive offices of the Company is 7 Times Square, 43rd Floor, New York, NY 10036.
Item 2.	Identity and Background
(a)	This Schedule 13D is being filed jointly by (1) Fairmount Funds Management LLC, a Delaware limited liability company and Securities and Exchange Commission registered investment adviser under the Investment Advisers Act of 1940 ("Fairmount"); (2) Fairmount Healthcare Fund II L.P., a Delaware limited partnership ("Fund II"); (3) Fairmount SPV III, LLC, a Delaware limited liability company ("SPV"); (4) Peter Harwin; and (5) Tomas Kiselak (Mr. Harwin, Mr. Kiselak, Fairmount, Fund II, and SPV are collectively referred to herein as the "Reporting Persons"). The joint filing agreement of the Reporting Persons is attached as Exhibit 99.1 to this Schedule 13D. Fairmount serves as investment manager for Fund II and the Class A member for SPV and may be deemed a beneficial owner for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the "Act") of any securities of the Company held by Fund II and SPV. Fund II has delegated to Fairmount the sole power to vote and the sole power to dispose of all securities held in Fund II's portfolio, including the shares of Common Stock reported herein. Because Fund II has divested itself of voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, Fund II disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act and therefore disclaims any obligation to report ownership of the reported securities under Section 13(d) of the Act. As the Class A member for SPV, Fairmount has the sole power to vote and the sole power to dispose of all of the securities held in SPV's portfolio, including the shares of Common Stock reported herein. Because SPV has no voting or investment power over the reported securities it holds, SPV disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act and therefore disclaims any obligation to report ownership of the reported securities under Section 13(d) of the Act. As managers of Fairmount, Mr. Harwin, and Mr. Kiselak may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Company beneficially owned by Fairmount. Fairmount, Mr. Harwin and Mr. Kiselak disclaim beneficial ownership of the securities reported in this Schedule 13D other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13D shall not be deemed an admission that any of Fairmount, Mr. Harwin or Mr. Kiselak is the beneficial owner of such securities for any other purpose.
(b)	The principal business address of each of the Reporting Persons is 200 Barr Harbor Drive, Suite 400, West Conshohocken, PA 19428.
(c)	The principal business of Fairmount is to provide discretionary investment management services to qualified investors through Fund II and SPV, which are each private pooled investment vehicles. The principal occupation of Mr. Harwin and Mr. Kiselak is investment management.
(d)	During the last five years, none of the Reporting Persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).
(e)	During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceedings was or is subject to a judgment, decree, or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violations with respect at such laws.
(f)	Fairmount is a Delaware limited liability company. Fund II is a Delaware limited partnership. SPV is a Delaware limited liability company. Mr. Harwin is a citizen of the United States of America. Mr. Kiselak is a citizen of Slovakia.
Item 3.	Source and Amount of Funds or Other Consideration
	Item 3 of the Statement is hereby amended and supplemented as follows: In aggregate, the Reporting Persons have voting and dispositive power over 4,052,052 shares of Common Stock of the Company, which is comprised of (a) 3,307,191 shares of Common Stock, and (b) Pre-Funded Warrants to purchase up to 744,861 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. On September 11, 2025, Fund II purchased a total of 600,000 shares of Common Stock from the Company for an aggregate of $19,800,000 in an underwritten public offering. The shares were purchased with working capital.
Item 4.	Purpose of Transaction
	Item 4 of the Statement is hereby amended and supplemented as follows: Fund II purchased the shares of Common Stock referenced in Item 3 for investment purposes. On March 4, 2025, Mr. Kiselak resigned from the Board of Directors of the Company. Mr. Kiselak's resignation was not the result of any disagreement with the Company on any matter relating to the Company's operations, policies or practices. As a result of his resignation, Mr. Kiselak no longer may be deemed to have influence over the corporate activities of the Company, including activities which may relate to items described in subparagraphs (a) through (j) of Item 4 of Schedule 13D. The Reporting Persons do not have any present plans or proposals that relate to or would result in any of the actions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D, although, the Reporting Persons, at any time and from time to time, may review, reconsider and change their position and/or change their purpose and/or develop such plans and may seek to influence management or the board of directors of the Company with respect to the business and affairs of the Company and may from time to time consider pursuing or proposing such matters with advisors, the Company or other persons.
Item 5.	Interest in Securities of the Issuer
(a)	The information in rows 11 and 13 of each of the cover pages of this Amendment No. 3 is incorporated by reference herein. The percentages used in this Schedule 13D are calculated based upon 40,561,085 shares of Common Stock outstanding as of September 11, 2025, consisting of (i) 32,188,345 shares of Common Stock outstanding as of August 6, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 7,627,879 shares of Common Stock issued in the Company's underwritten public offering on September 11, 2025, as reported in the Company's Current Report on Form 8-K filed on September 11, 2025, and (iii) the 744,861 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation.
(b)	The information in rows 7 through 10 of each of the cover pages of this Amendment No. 3 is incorporated by reference herein. The Reporting Persons' securities consist of (a) 3,307,191 shares of Common Stock, and (b) Pre-Funded Warrants to purchase up to 744,861 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation.
(c)	On September 11, 2025, Fund II purchased a total of 600,000 shares of Common Stock from the Company at a price of $33.00 per share, for an aggregate of $19,800,000 in an underwritten public offering.
(d)	Fairmount is the investment manager to Fund II and the Class A member for SPV and has voting and dispositive power over shares of Common Stock held on behalf of Fund II and SPV.
Item 7.	Material to be Filed as Exhibits.
	Exhibit 99.1 Joint Filing Agreement

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Fairmount Funds Management LLC

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	09/15/2025

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	09/15/2025

Fairmount Healthcare Fund II L.P.

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	09/15/2025

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	09/15/2025

Fairmount SPV III, LLC

Signature:	By: Fairmount Funds Management LLC, its Class A Member, /s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	09/15/2025

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	09/15/2025

Peter Evan Harwin

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin
Date:	09/15/2025

Tomas Kiselak

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak
Date:	09/15/2025

FAQ

How many Dianthus Therapeutics (DNTH) shares do Fairmount reporting persons control?

They report voting and dispositive power over 4,052,052 shares, equal to 9.9% of the outstanding common stock as of September 11, 2025.

What transaction did Fairmount Healthcare Fund II complete on September 11, 2025?

Fund II purchased 600,000 shares in the company’s underwritten public offering at $33.00 per share for a total of $19,800,000.

Do the reporting persons hold any warrants in DNTH?

Yes. The position includes pre-funded warrants exercisable for up to 744,861 shares, but exercise is subject to a 9.99% beneficial ownership limitation.

Did any Reporting Person resign from Dianthus’ Board?

Yes. Tomas Kiselak resigned from the Board on March 4, 2025, and the filing states the resignation was not due to any disagreement with the company.

Do the Reporting Persons plan to seek control of Dianthus?

The filing states the Reporting Persons do not have any present plans or proposals